Metal contamination is one of the factors that cause deterioration in semiconductor device characteristics. For example, in the case of a back-illumination solid-state image sensor, metal that is mixed into a semiconductor epitaxial wafer used as a substrate of the image sensor is a factor causing increased dark current in the solid-state image sensor and causing defects referred to as white spot defects. In a back-illumination solid-state image sensor, a wiring layer and the like are provided in a lower layer than a sensor section such that external light can be directly taken in by the sensor, enabling clear images and videos to be recorded even in dark locations. For this reason, back-illumination solid-state image sensors have become widely used in recent years in digital video cameras and mobile telephones such as smart phones. Therefore, it is desirable to reduce white spot defects to as great an extent as possible.
Mixing of a metal into a wafer mainly occurs during a process of producing a semiconductor epitaxial wafer and a process of producing a solid-state image sensor (device production process). Metal contamination in the former process of producing a semiconductor epitaxial wafer may for example occur due to heavy metal particles from constituent materials of an epitaxial growth furnace or heavy metal particles produced through metal corrosion of piping materials as a result of a chlorine-containing gas being used in the furnace during epitaxial growth. The metal contamination described above has been improved to a certain extent in recent years through replacement of the constituent materials of epitaxial growth furnaces with materials having superior corrosion resistance; however, this improvement is still insufficient. On the other hand, in the latter process of producing a solid-state image sensor, heavy metal contamination of a semiconductor substrate is a concern during various processing steps such as ion implantation, diffusion, and oxidizing heat treatment.
One technique for inhibiting heavy metal contamination such as described above involves providing a gettering site in a semiconductor wafer for capturing heavy metals. In a known method using this technique, ions are implanted in a semiconductor wafer and an epitaxial layer is subsequently formed. In this method, a region into which the ions are implanted functions as a gettering site.
PTL 1 describes a semiconductor epitaxial wafer production method including: a first step of irradiating a surface of a semiconductor wafer with cluster ions to form a modified layer that is located in a surface portion of the semiconductor wafer and that includes a constituent element of the cluster ions in solid solution; and a second step of forming an epitaxial layer on the modified layer of the semiconductor wafer.